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OPEN Urban storm water infltration systems are not reliable sinks for biocides: evidence from column experiments Marcus Bork1,2*, Jens Lange1, Markus Graf‑Rosenfellner2, Birte Hensen3, Oliver Olsson3, Thomas Hartung2, Elena Fernández‑Pascual3,4 & Friederike Lang2

Groundwater quality in urban catchments is endangered by the input of biocides, such as those used in facade paints to suppress algae and fungal growth and washed of by heavy rainfall. Their retention in storm water infltration systems (SIS) depends, in addition to their molecular properties, on chemical properties and structure of the integrated soil layer. These soil properties change over time and thus possibly also the relevance of preferential fow paths, e.g. due to ongoing biological activity. To investigate the mobility of biocides in SIS, we analyzed the breakthrough of diferently adsorbing tracers (bromide, uranine, sulforhodamine B) and commonly used biocides (diuron, terbutryn, octhilinone) in laboratory column experiments of undisturbed soil cores of SIS, covering ages from 3 to 18 years. Despite similar soil texture and chemical soil properties, retention of tracers and biocides difered distinctly between SIS. Tracer and biocide breakthrough ranged from 54% and 5%, to 96% and 54%, respectively. We related the reduced solute retention to preferential transport in macropores as could be confrmed by brilliant blue staining. Our results suggest an increasing risk of groundwater pollution with increasing number of macropores related to biological activity and the age of SIS.

Urban storm water management concepts such as green infrastructure (GI) have become increasingly imple- mented worldwide for their numerous environmental benefts­ 1–4. One example of GI is local infltration of urban storm water that, in Germany, is required by the German federal water ­law5 due to benefts for micro-climate6 and urban hydrology such as reducing storm water ­quantity7, 8, restoring groundwater ­levels9–11 and relieving sewer ­systems12. However, the direct infltration of urban storm water could lead to groundwater pollution since storm water may be loaded with nutrients­ 13, heavy ­metals14, 15 and organic pollutants such as ­hydrocarbons16, 17 and pesticides­ 18–20. Recently, biocides used in facade renders and paints to suppress algae and fungal ­growth21 are receiving increasing attention as they can be washed of from facades by heavy rainfall­ 22–25. Tis biocide wash-of occurs during entire rain events­ 26, depends on the conditions of the rainfall and on transport processes within the facade material­ 27 and happens even fourteen years afer painting of the ­facade28. Biocide-loaded storm water either infltrates into the soil around houses­ 29 or is directed into SIS such as swales and swale-trench-systems. Together with local infltration of storm water, the purpose of SIS is to reduce moderate contaminant ­load30, 31. Reviews ­of17 and­ 32 showed that this may be the case for hydrocarbons or pes- ticides. ­However28, recently showed that SIS could be pathways for biocides and their transformation products into groundwater. Consequently, further research is needed on factors that may favor the leaching of organic pollutants in SIS. Generally, the uppermost layer of swales and swale-trench-systems consists of topsoil material­ 33. In these layers, organic pollutants such as biocides may be retained by similar processes as in natural soils: transforma- tion, mineralization, plant uptake and sorption (in the following we will call the uppermost soil flter media of SIS only soil). Tese processes depend on molecular properties of the solutes, on the vegetation cover and on the physical and chemical soil properties such as pH, organic matter (OM) content, texture and structure­ 34, which

1Hydrology, Faculty of Environment and Natural Resources, University of Freiburg, 79098 Freiburg, Germany. 2Soil Ecology, Faculty of Environment and Natural Resources, University of Freiburg, 79098 Freiburg, Germany. 3Institute of Sustainable and Environmental Chemistry, Leuphana University Lüneburg, 21335 Lünbeburg, Germany. 4Environmental Research Institute, University College Cork, Cork T23 XE10, Ireland. *email: [email protected]

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− Figure 1. Depth-dependent soil properties: (a) stone content [% (w/w)], (b) bulk density ( g cm 3 ), (c) pH (0.01 M CaCl2 ) and (d) organic carbon content (OC) [% (w/w)] of the three sites F3, W.10, V18. Te error bars are the standard deviation ( n = 4).

change with time. Furthermore, the retention can be limited by the formation of preferential fow paths. Evidence regarding the relevance of these diferent processes is missing so far. Typically, soil pores are clogged by the entry of suspended solids with storm water­ 35 thus reducing water infltration. Nevertheless, the hydraulic efciency of SIS has been reported to be maintained even afer long- term operation­ 8, 33, 36. To the best of our knowledge, no study has systematically addressed this contradiction so far. One explanation could be an increased number of preferential fow paths due to root growth and other biological activity that compensates for soil pore clogging. Yet preferential fow paths induce solute transport­ 37 as observed for pesticides in agricultural soils­ 38, 39. Tus, we investigated three SIS with regard to the infuence of preferential fow paths on the transport of biocides. We analyzed the depth distributions of physical and chemical soil properties and conducted laboratory- scale percolation experiments using undisturbed soil core samples from three SIS established 3, 10 and 18 years ago (F.3, W.10 and V.18). In these experiments we investigated the breakthrough of three commonly used ­biocides29, 40: diuron (3-(3,4-dichlorophenyl)-1,1-dimethylurea), terbutryn (N2-tert-butyl-N4-ethyl-6-methyl- sulfanyl-1,3,5-triazine-2,4-diamine), and OIT (2-octyl-1,2-thiazol-3-one). Furthermore, we applied a tracer − − mix of non-adsorbing bromide ( Br ) and chloride ( Cl ), together with the variously adsorbing fuorescent dyes uranine (UR) and sulforhodamine B (SRB) that are ofen used to investigate preferential ­fow41–44. With this multi-tracer approach and a staining of the soil column using brilliant blue, we aimed to investigate pollutant retention capacity of diferent SIS. Results and discussion Soil properties. Stone content. Te stone content ranged from 15 ± 8% (w/w) at V.18 to 44 ± 13% (w/w) at F.3 (Fig. 1a, Table 1). Tese diferences between sites may partly be due to diferent sources of the raw material used to create the SIS. Further, the stone content increased with depth within the frst 15 cm (V.18) and 10 cm (W.10), but remained approximately constant over depth at F.3. Hence, the stone content in the upper layers of the older SIS (W.10 and V.18) was lower than in the lower layers. Tese depth-related diferences at each site may be related to time-dependent developments within the SIS. In the uppermost layers of V.18 and W.10, stone content was comparatively low probably due to input of fne mineral and organic particles by storm water. For

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Sand Silt Clay − Site.age Stones [% (w/w)] pHa (–) OC [% (w/w)] Bulk density ( g cm 3) [% (w/w)] F.3 44 ± 13 7.5 ± 0.1 1.1 ± 0.2 1.6 ± 0.2 80 ± 2 16 ± 2 5 ± 1 W.10 34 ± 10 7.2 ± 0.1 1.6 ± 0.8 1.4 ± 0.2 73 ± 4 20 ± 3 7 ± 1 V.18 15 ± 8 7.1 ± 0.1 1.9 ± 1.2 1.3 ± 0.2 57 ± 7 34 ± 6 9 ± 2

Table 1. Soil properties of SIS. Tey are calculated as the mean of all depth steps of four soil cores ± standard deviation. Since the number of depth steps difered between the sites and the number of laboratory repetitions difered, the number of repetitions varied among the methods and the sites (pH: n = 51–69, OC: n = 17–23, a Stones: n = 16–22, Bulk density: n = 16–22; for details see Table S4). pH in 0.01 M CaCl2.

the oldest SIS (V.18), this assumption is supported by the feld observation of soil material lying on a bricked stone border near the infow within the SIS.

Bulk density. Te bulk density in the upper layers of the diferent SIS increased in the following order: V.18 − < W.10 < F.3 (Fig. 1b, Table 1). At V.18, we observed the strongest change with depth from 1.0 ± 0.1 g cm 3 − (0–5 cm) to 1.5 ± 0.1 g cm 3 (15–20 cm). In contrast, we observed almost no depth-dependent change of bulk − density at the youngest site of F.3 (1.6 ± 0.2 g cm 3). In samples of the older sites of V.18 and W.10, low bulk densities in the uppermost layers compared to deeper layers were probably caused by the activity of macrofauna, an intensive rooting, a higher organic carbon (OC) content and the input of strongly sorted fne material. Te older the SIS, the stronger the efect of these factors. At F.3 the bulk density was relatively high. Here, we supposed an uniform compaction of the soil layer under the topsoil during construction. Tis assumption was supported by the observation of redox characteristics (iron-red stains next to grey iron-depleted areas) in the soil at approximately 25 cm depth caused by the lack of oxygen due to accumulating water­ 45 in compacted soil.

Texture. Te mean texture of fne soil at all SIS was very similar: 57–80% (w/w) sand, 16–34% (w/w) silt and 5–9% (w/w) clay, since similar textured materials were used for construction to guarantee solute retention and sufcient hydraulic conductivity­ 31. Average clay contents of all SIS were within acceptable ranges of Best Man- agement Practice (BMP) claimed by ATV-DVWK A-138 (< 10% (w/w) clay)31. Te clay fraction at the oldest SIS (V.18) decreased slightly with depth from 12% (w/w) at 0–5 cm to 7% (w/w) at 20–25 cm (Figure S4) while it remained constant with depth at W.10 and F.3. Since a homogeneous texture afer SIS construction can be assumed for each SIS, a higher clay content in the uppermost layers suggested fne particle deposition by storm water, as was observed by­ 46.

pH. Te pH was above 7.0 at nearly all sites and depths (Fig. 1c, Table 1) and was thus within the range of ­BMP31. Te pH increased slightly in the following order: V.18 < W.10 < F.3. Tese slight pH diferences among the SIS could be either due to age (stronger acidifcation of older soils) or due to diferent initial pH values of the topsoil materials. Furthermore, the pH increased with depth at all sites, which was stronger at older sites (V.18 and W.10) than at F.3. Tese larger depth-gradients at older sites indicate a relationship between pH and age of the sites: older SIS were more acidifed in the upper layers than the youngest SIS (F.3) due to the input of acids by precipitation, 45 the production of CO2 by soil respiration and the release of protons and organic acids by ­roots .

Organic carbon content. Te OC content and the slope of depth-gradients within the upper 15 cm increased from 1.15 ± 0.3% (w/w) at the youngest SIS to 2.85 ± 0.9% (w/w) at the oldest SIS (Fig. 1d, Table 1). Tis indi- cates that, in general, OC accumulated during soil development in upper soil layers while the stone content, the bulk density and the pH decreased. Overall, the OC contents of all SIS were within the BMP ranges­ 31.

Soil development and spatial variability. All soil parameters described above and especially the development of depth gradients indicate continuous soil development (alteration of important soil properties such as pH, texture, OC content and distribution) in the investigated SIS due to physical and chemical processes and sedi- ment input with storm water. Consequently, biological activity also increased, as shown by the observation of macrofauna at V.18 (earthworms) and W.10 (ants) while no macrofauna was observed at F.3. Overall, age of SIS is refected in the soil properties afer only a few years. We assumed that the spatial variability of the soils within each SIS was relatively small, as probably only one material was used to build up the upper soil layer. As described above, a low spatial variability within each SIS was confrmed by the low variability of analyzed soil properties of the soil samples ( n = 4 ). Terefore, we assumed in the present study that one soil column with a wide diameter (20 cm) at each SIS can be considered representative of the SIS infow area. To determine the infuence of fow paths on solute transport we thus considered a column experiment without repetition as sufcient to further investigate the diferences between the SIS. Tis is common for such complex column experiments with undisturbed soils (see e.g.47–49) since their performance is very time-consuming.

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qavc qmind qmaxc Kse

a b −1 Column (site.age) dcol (cm) Lcol (cm) PVtot,est (L) Time (h) TPS (L) (cm h ) F.3 20 21 1.29 28.0 5.2 0.59 0.32 0.97 0.18 W.10 20 25 1.91 12.7 7.6 1.91 1.58 2.60 0.66 V.18 20 25 2.80 12.7 11.2 2.82 2.40 3.70 0.98

Table 2. Soil column properties, water fuxes q [average (av), minimum (min) and maximum (max)] and total percolated water (L) during the column experiment in the three soil columns F3, W.10 and V.18. a 50 PVtot,est : estimated total pore volume: estimated from pedotransfer functions according to­ and corrected by stone content (see Eq. 1). b TPS: total percolated biocide/tracer solution (L) afer the percolation of four c pore volumes. qav is the average water fux through the soil column and is the slope of the linear regression of the outfow (L) over time (h). d q and q are the lowest and the highest calculated water fux during the min max − column experiment. e Calculated according to Eq. (2) with q and a hydraulic head gradient of 3.2 cm cm 1 − av (F.3) and 2.9 cm cm 1 (W.10 and V.18).

Percolation experiment. − Saturated hydraulic conductivity and water fux. Te saturated hydraulic con- cm h 1 ductivity K− s ( ) difered strongly between the− soil columns (Table 2). It was lowest in the youngest SIS F.3 0.18 cm h 1 0.66 cm h 1 ( − ), four times higher at W.10 ( ), and approximately six times higher in the oldest SIS V.18 0.98 cm h 1 ( ). Water fow velocities− at all sites− and time steps were in the optimal range for urban swale systems defned by BMP ( 0.006 cm h 1 < q < 6 cm h 1)31. Te water fux decreased continuously with time in all soil columns: in W.10 and V.18 approximately 1.5 times and in F.3 approximately 3 times (Table 2, Figure S7 and Fig. S8). Tis could be due to clogging of pores in the 0.45 µm nylon membrane by colloids­ 51 or dissolved organic matter­ 52. In our experiment, the existence of colloids was likely as they could be mobilized by a decreased ionic strength of the infltrating deionized water­ 53.

− − Tracer breakthrough. Breakthrough curves (BTCs) of Br and Cl reached 90–100% at the outlet of all soil columns afer percolation of four pore volumes (PV) (Fig. 2). Both ions showed the fastest breakthrough of all tracers suggesting only a weak interaction with the soil matrix; as in other studies, they can be considered as con- servative ­tracers54, 55. Nevertheless, in our experiment, this assumption was challenged by the observation in the − − youngest SIS (F.3): afer the percolation of one PV, only 30% Br and Cl broke through. Under the assumption − − that the transport of Br and Cl is only determined by convection, difusion and dispersion, 50% of the solutes should have reached the outlet afer one PV was exchanged­ 56. Te 20% defcit indicated that weak adsorption − − of Br and Cl occurred in F.3 as was also observed by other researchers, e.g.57. In contrast, at W.10 and V.18 − − substantially more than 50% Br reached the outlet afer one PV, indicating fast transport of Br in macropores. However, since the PV of the individual soil columns are only rough estimates, they are subject to uncertainty. Terefore, the deviation from 50% solute breakthrough at a PV equals one could also be partly explained by the uncertainties in the calculation of the PV. With the exception of UR at V.18, BTCs of UR and SRB did not reach 100% afer four PV. Instead, their breakthrough maxima were 51 ± 2% (F.3) and 79 ± 1% (W.10) for UR; and 25 ± 1% (F.3), 70 ± 1% (W.10), and 90 ± 2% (V.18) for SRB. Maxima of BTCs at F.3 and W.10 increased in the following order: − − − − SRB < UR < Br ≈ Cl . At V.18, this order slightly difered (SRB < UR ≈ Br ≈ Cl ). Tese results indicated − − a stronger retention of the fuorescent tracers compared to Br and Cl most likely due to their higher adsorp- tion afnity­ 58. Te higher retention of SRB compared to UR was in accordance with literature­ 59, 60 and linear sorption coefents (K d-values) for SRB that were about twice as high compared to UR (Table S1 and Figure S11) − − for all SIS. However, the high discrepancy between the retention of conservative tracers ( Br , Cl ) and UR at F.3 revealed that UR ought to be considered as a non-conservative tracer with respect to its sorption properties. Tis was described previously for UR application in soils with comparable OC content, texture and pH­ 44, 61, 62. We observed a faster breakthrough of all substances in W.10 and V.18 than in F.3 as indicated by higher slopes and higher maxima of BTCs (Fig. 2). Afer the percolation of one PV, 29% (F.3), 73% (W.10), and 93% (V.18) of − the Br infow concentrations were reached. Due to the small diferences between the chemical soil properties of the SIS and the Kd values of UR and SRB (Table S1), we would have expected similar tracer BTCs. Yet, this was not the case; tracer retention decreased with age from the youngest to the oldest SIS (F.3 << W.10 < V.18). Tese diferences could not be explained by chemical soil properties alone. When they would dominate solute transport, sorption, and thus solute retention should be highest at V.18 where OC content (1.9%) and clay content (8.8%) were highest and pH was lowest (7.1) (Table 1). From this we deduce that solute retention in W.10 and V.18 was controlled less by sorption than by preferential fow in macropores. Existing studies support this fnding as they related fast transport of SRB­ 42, 43 and UR­ 44 in undisturbed soil cores to the existence of preferential fow, too. Furthermore, our results imply the following: when preferential fow in macropores dominates water and solute transport, the diferences in the BTCs of substances with diferent adsorption afnities decrease. Tis efect led to the largest diferences between tracer BTCs in F.3, where matrix fow was dominant, and to the smallest diferences between tracer BTCs in V.18, where preferential fow was dominant. In F.3, the matrix fow was high and solutes had a longer contact to a larger number of sorption sites thus intensifying sorption losses. In V.18, the opposite was true: the contact of the solutes to sorption sites was brief as they bypassed the soil matrix with the preferential fow. Additionally, the number of sorption sites may be lower in the preferential fow regions­ 42.

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Figure 2. Breakthrough curves (BTCs) of the tracers bromide, chloride, UR, SRB and the biocides diuron, terbutryn, OIT and the sum of biocides (diuron+terbutryn+OIT; represented by diamonds). Te normalized concentrations c/c0 (%) in the outfow from soil columns was plotted against the pore volume (–).

Figure 3. Brilliant blue stained soil columns (F.3, W.10 and V.18).

Furthermore, similar BTCs of conservative and non-conservative tracers with diferent adsorption afnities indicate strong transport in macropores by preferential fow­ 63–65.

Brilliant blue staining. Roughly 65% of F.3, 38% of W.10 and 8% of V.18 (Figure S10) were stained. Along the cut profle large blue patches were visible for F.3 while only single blue fngers were observable for V.18 (Fig. 3). Tese macropores, especially in V.18, were most probably caused by ­earthworms45, 66. In fact, approximately ten

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Table 3. Characterisation of fuorescent tracers and biocides. aColor Index (Name:Number): UR (Acid Yellow 58 58 b 73 : 45350) ; SRB (Acid Rd 52 : 45100) ; log(KOW ) for the twofold protonated (neutral) species (dominant species between pH 1.95 and 5.05), Calculated with Estimation Programs Interface Suite for Microsof 99 c Windows, v 4.11 ; log(KOW ) for the disodium salt (twofold negative charged) species (dominant species above 99 d pH 7.00), Calculated with Estimation Programs Interface Suite for Microsof Windows, v 4.11 ; log(KOW ) at pH 7.15100; eSource:58; f Source:94; gSource:101; hSource:102; i Source:103; jSource:29.

individuals were observed on the laboratory columns afer saturation from bottom to top. Earthworm holes were visible on the soil surface as well as on horizontal and vertical cuts through the soil column (approximately 2–3 mm, Figure S9). At W.10 several ants were observed that also could cause ­macropores67, 68. In contrast, no macrofauna was visible at F.3. Overall, brilliant blue patterns supported the assumption of an increasing fraction of preferential fow in the older SIS W.10 and V.18. Brilliant blue staining also revealed partial water fow on the soil column sidewall­ 69 that bypassed a part of the soil column F.3 and entered the soil again further down (Fig. 3). Tis partial sidewall fow could potentially − − explain the fast initial Br , Cl and UR breakthrough in F.3 where 5–10% of the tracers was measured in the frst sample taken afer 30 min (Fig. 2). However, this efect was limited since concentrations remained only initially constant and increased again afer approximately 0.5 PV. Interestingly, this efect could not be observed for the stronger adsorbing SRB. Possibly, SRB was initially retained and therefore could not be measured in the frst percolates. Similar transport characteristics of UR and SRB in the beginning of the experiment were observed e.g. in the study conducted ­by70.

Biocide breakthrough. At all sites, the biocide breakthrough increased in the following order: OIT < terbutryn < diuron. Tis order followed the polarity of the substances as indicated by the KOW-values (Table 3) which can be interpreted as a rough estimate for the sorption afnity of non-polar substances to organic matter­ 71, 72. Further- more, except for diuron at W.10 and V.18, biocides were more strongly retained than the tracers in all SIS. Tis observation may be explained as follows: the walls of macropores caused by earthworms may be enriched with soil organic ­matter73. Tese macropores may provide hydrophobic sorption sites (organic matter) for non-polar substances such as biocides and less for the polar tracers. Analogous to UR and SRB, the retention of biocides was highest in F.3 (Fig. 2), followed by W.10 and V.18 and was negatively related to the OC content of all SIS (Table 1). Similar to the tracers, the large diferences in BTCs of biocides between the SIS cannot be explained by chemical soil properties but rather by soil structure. In F.3, biocides had more intense contact with the soil matrix than in W.10 and V.18 due to a higher fraction of matrix fow. As a result, biocide retention was highest in F.3 despite the lowest OC content. Tus, the infuence of biocide properties on their retention was strongly reduced by preferential fow in macropores­ 74, 75. Although biocide degradation is unlikely due to the short duration of the experiment (approximately 13–28 h) and due to immediate cooling and freezing of the samples afer sampling, these processes could not be completely ruled out. However, if a small amount of biocide degradation occurred, then concentrations would have been reduced by the same amount in all samples. Te diferences in biocide concentrations between the individual soil columns should not have been afected. Te rapid leaching of organic pollutants through macropores, has also been observed in several studies for pesticides in agricultural ­soils38, 39. More biocide breakthrough in W.10 and V.18, especially that of diuron in V.18 is in line with fndings ­of76, who reported leaching of diuron due to complexation of diuron with dissolved organic matter (DOM). Tis indicates that DOM molecules have a dual role on pollutant transport in soils: they compete for adsorption of the organic pollutants and enhance their mobility by complexation. But their leaching is enhanced by preferential transport in macropores following intense rainfall­ 77 or in strongly structured soils with connection to shallow groundwater­ 78.

Important factors of fast solute breakthrough in urban SIS and implications for storm water treatment. Despite similar chemical soil properties between the SIS, W.10 and V.18 showed a much faster tracer and biocide breakthrough compared to F.3. Brilliant blue patterns and saturated hydraulic conductivities indicated prevailing preferential fow conditions in macropores in SIS W.10 and V.18 that lead to fast solute

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breakthrough. Macropores can be caused by biological ­activity41, 79, which is supported by the observation of the highest activity of macrofauna (earthworms and ants) in the older SIS W.10 and V.18. Terefore, in our experi- ment the age of SIS is closely related to macrofauna activity. Tis observation is in agreement with results from­ 80 who found an increasing earthworm population density with increasing age of diferent urban landscapes. Simi- lar observations were made by­ 81 in technosols but they concluded that the existence of an initial addition of topsoil is required, which was the case in all SIS of our study. Beside the age of a soil, the abundance of earthworms is also controlled by bulk density and soil ­depth82. In SIS V.18 bulk density was lowest and soil depth highest, both factors favoring high earthworm abundance. Tus, diferent relevance of preferential fow in the diferent SIS may also be due to diferences in the construction of the SIS. If a SIS is built with a less thick topsoil layer or a high bulk density, or if other macrofauna-inhibiting factors play a role, the development of the macrofauna may be restricted, making the development of preferential fow paths in aging SIS less likely. Furthermore, climate can infuence the population dynamics of earthworms­ 83, 84. Terefore, we conclude that factors supporting higher activity of macrofauna (e.g. earthworms and ants) in SIS may also lead to faster solute breakthrough. Additional infuence on macropore formation can be exerted by the vegetation of the swales, directly by plants with thicker or deeper roots colonizing as development of the swales progresses, or indirectly by infu- encing the diversity of invertebrates­ 85. Due to necessary time for colonization and establishment of species also the age of swales may be linked to the biodiversity. In addition to vegetation and soil structure, the shape of the swales could also have an infuence on diversity of invertebrate and thus on macropore formation. ­Reference85 supposed that biodiversity may be higher in round systems than in linear ones. However, we observed rather the opposite, suggesting that age played a greater role for macrofauna diversity and macropore formation than swale shape in our experiments. Our experiment has shown that SIS can retain biocides by adsorption when the substances have sufcient contact with the soil matrix. Inversely, our study also suggests a decrease in biocide retention capacity in urban SIS due to preferential fow pathways caused by an increasing biological activity and changing soil properties already afer 10–18 years of operation. However, biological activity in SIS is also desirable due to several benefts. Higher biodiversity of vegetation and soil fauna may enhance degradation of several organic pollutants and higher infltration rates, e.g. due to a higher number of macropores, may guaranty fast water infltration even afer long-term operation. Overall, we recommend regular monitoring of the pollutant retention capacity of SIS to detect its reduction in time, which could be done, for example, by tracer experiments. One approach to address this problem is to treat storm water with special adsorbent materials­ 86, 87 before it enters the swales, or integrate additional adsorbent layers in the swales. However, these options are complex and expensive and, furthermore, a large portion of urban runof ofen infltrates difusely and does not reach the swales at all. Terefore, a more sustainable approach, is to avoid biocide pollution at the source­ 88, 89, which would allow a targeted urban water management by SIS that preserves urban groundwater quality. Methods Study sites and soil characterization. We selected three SIS of diferent age (F.3, W.10 and V.18) in the city of Freiburg, south-west Germany (Figure S1). F.3 (3 years old) was a nearly rectangular shaped swale (approx. 600 m2 ) that drained a commercial area, W.10 (10 years old) a rectangular multilevel swale system (approx. 700 m2 ), and V.18 (18 years old) an elongated swale-trench system (approx. 3000 m2 ), both drained a residential area. F.3 consisted of a 25–30 cm topsoil layer over the natural soil layer. Beneath a 30–50 cm top- soil layer of W.10 and V.18 there was a 20 cm sand layer followed by a gravel-flled drain trench to collect and drain the seepage water. To ensure comparability despite diferent SIS geometry, all samples were taken in the intermediate vicinity of the infow and from similar depths (20–25 cm). Moreover, the infow area is particularly interesting because it represents the entry point of pollutants in the SIS. Te vegetation of the swales consisted mainly of grass as well as clover (Trifolium), dandelion (Taraxacum) and ribwort (Plantago). From each SIS, four fxed volume soil cores (diameter: 8 cm, length: 15 cm) were taken with a root auger in a 2 × 2 m square at two depths (0–15 cm and 15–30 cm). Tis was performed in two steps (Figure S2): afer taking the upper soil core (0–15 cm), the deeper one was taken from the same bore hole (15–30 cm). Using a knife, each soil core was cut into 5 cm wide pieces resulting in fve (F.3, W.10) and six (V.18) depth- related soil samples (0–5 cm, 5–10 cm, 10–15 cm, 15–20 cm, 20–25 cm, 25–30 cm; n = 4 ). Soil samples were air-dried and weighed to calculate bulk density. Subsequently, the soil samples passed through a 2 mm sieve and stones (inorganic particles > 2 mm ) and roots were weighed separately. We determined the residual gravimetric water content as the diference between the weight of an air-dried soil sample before and afer 24 h of drying at ◦ 105 C . Te pH was measured from over-night, 25 ml 0.01 M CaCl2 supsensions of 10 g soil at room temperature ◦ ◦ ( 23 C ± 2 C ) with a pH meter (Deutsche METROHM GmbH & Co KG, Filderstadt, Germany). Te content of sand, silt and clay (inorganic particles < 2 mm ; fne soil) were determined by sieving, sedimentation and the pipette ­method90. Te OC content was determined by a CNS-analyzer (vario EL cube, Elementar Analysensys- teme GmbH, Germany). Due to the possible carbonate content ( pH > 7.0 ), the OC content of the soil samples ◦ was determined as the diference of total carbon before and afer heating at 550 C . At this temperature OC had 91 been transformed to CO2 . Additionally, sorption isotherms for UR and SRB were produced according to OECD 92 62 guideline ­106 described in­ and linear sorption coefents ( Kd-values) were calculated.

Percolation experiment. In every SIS, one intact soil column was collected using a stainless steel cylinder (diameter: 20 cm, length: 30 cm) that was knocked into the soil. Te cylinder was excavated and the embedded soil column was pushed into a second steel cylinder (Figure S3). Te bottom of the soil column was straightened with a knife and approx. 1–2 cm sand was added to ensure connection to a 0.45 µm nylon membrane. Te height

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of soil column F.3 was lower (21 cm) than of W.10 and V.18 (25 cm) because the soil layer of F.3 was shallower. During sampling, the vegetation was lef on the soil column as far as possible. Where it was too long, it was superfcially shortened. Plants were not removed so as not to disturb the soil structure. Te top of the soil column was connected to two storage vessels (Figures S5, S6). Te frst storage vessel was connected via a tube with a small liquid layer on the top of the soil (approximately 2 cm). Te second storage vessel was connected to the frst one with two tubes that kept the water level between the storage vessels and the soil column constant. At the bottom of the soil column, the solution was placed under tension with a 45 cm water head to quicken fow rates and thus reduce anaerobic conditions in the soil column.Te bottom of the column was enclosed with a 0.45 µm nylon membrane. Te percolate was collected in 1 L-glass bottles that were placed on weighing devices (Figure S5) to calculate the fow rate. To avoid photodegradation, all vessels, tubes and bottles were wrapped− with aluminum foil. Te soil columns were saturated with deionized water from bottom to top at 7 cm d 1 . Tereafer, deionized water was percolated to reach constant fow conditions and to decrease − the DOC load of the percolate. Pre-tests with Br (NaBr, Carl Roth GmbH & Co KG, Karlsruhe, Germany) were − conducted to set up a sampling protocol for each soil column. Before starting the main experiment, Br was washed out by deionized water. Te main percolation experiment started with a 1.5 h fushing of the soil columns with deionized water. Subsequently, all water in the storage vessels and on the soil surface was replaced by a tracer/biocide solution. For each soil column, 15 L of initial solution consisting of deionized water, tracers and biocides were prepared. −1 − −1 − Target tracer concentrations were 50mgL Br , 25mgL Cl (CaCl 2 , VWR International GmbH, Darmstadt, − − Germany), 10mgL 1 UR (Simon & Werner GmbH, Flörsheim, Germany), 400 mg L 1 SRB (Chroma GmbH & Co KG, Münster, Germany). Target biocide concentrations are based on commonly measured concentrations − in facade runof­ 93 and were 50mgL 1 each of diuron, terbutryn (NEOCHEMA GmbH, Bodenheim, Germany) and OIT (Sigma-Aldrich Chemie GmbH, Taufirchen, Germany). Te tracer stock solutions were prepared with deionized water and solid substances, while the biocides were already dissolved in acetonitrile by the manu- facturer. UR and SRB solutions were stored in amber glass bottles and wrapped with aluminum foil to prevent photolytic decay. Te measured initial concentrations only slightly deviated from the intended concentrations (Table S5). Te experiment lasted for 28 h (F.3) and 12.7 h (W.10, V.18) depending on the fow velocity. We excluded bio- degradation of biocides since their half-times in soils are much higher than the duration of the experiment­ 29. Furthermore, terbutryn, diuron and OIT are assumed to be stable to aqueous hydrolysis­ 28, 94. In the beginning of the experiment, samples of the percolate were taken every 15 min, later every 20, 30, 45 or 60 min. An aliquot of the collected percolate was flled into 100 mL amber glass bottles for UR, SRB and biocide measurements, − − ◦ and 100 mL polyethylene bottles for Br , Cl and pH measurements. Samples were stored at approximately 6 C for measurement for a maximum of ten days and frozen for longer storage. No changes in concentrations were observed in preliminary laboratory tests measuring biocide concentrations before and afer storage (freezing of samples for multiple weeks). Te estimated total pore volume PVtot,est (L) of each soil column (Table 2) was calculated by the following: stone content (%) PVtot,est (L) = porosity (−) · 1 − · total volumn of the soil column (L). 100%  (1)

Te porosity (–) was estimated according to­ 50, who provide average porosities for soils in dependence of their −1 texture and OM content. Te estimated water volume that fowed through the column at time t (PV t LL ) was calculated by dividing the outfow (L) at a certain time step by PVtot,est . PVt was used to normalize the perco- lated amount of water and make solute transport comparable. Maxima of BTCs of the solutes were estimated by K calculating− the mean breakthrough (%) between PVs of three and four. Te saturated hydraulic conductivity s ( cm h 1 ) was calculated according to Darcy’s law: H q = Ks · . (2) L − where q ( cm h 1 ) is the water fow through the soil column, H (cm) is the hydraulic head diference between upper and lower boundary of the soil column, and L (cm) is the length of the column. Data analysis was per- formed with R statistics (version 3.3.4)95.

Brilliant blue staining. To identify preferential fow in soils of SIS we used brilliant blue staining as was done before e.g. by­ 42, 96. In each soil column about 3.5 L (approximately one PV) brilliant blue FCF (Waldeck − GmbH & Co KG, Germany) solution (c = 2gL 1 ) was applied. Due to diferent fow velocities, it took 23 h (F.3), 8.2 h (W.10) and 5.2 h (V.18) until the brilliant blue solution infltrated. Subsequently, we removed the soil column from the steel cylinders, cut them in half using a knife and photographed them (Canon 450D) to make fow pathways visible. Pictures were processed by Gimp 2.10 (Te GIMP team, www.​gimp.​org) and ImageJ-win 64 (Fiji Is Just ImageJ, fji.sc)97 (see Note S1). Te correlation between the brilliant blue stained area of the soil columns and the breakthrough maxima was tested with Pearson correlation coefcients.

Measurement of salt and fuorescent tracers. UR and SRB (Table 3) fuorescence was measured at 488 nm (UR) and 560 nm (SRB) in a synchronous scan method (wavelength range: = 250–650 nm, = 25 nm ) using the luminescence spectrometer LS-50B (Perkin Elmer, MA, USA). Due to sensitivity of tracer fuorescence to pH­ 98, it was bufered before measurement at 9–10 using one drop of 1.5 M EDTA to ensure 100% fuorescence intensity for both tracers. To ensure a linear calibration range, the calibration solutions were

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− − prepared in the ranges 0.25–5 µgL 1 (UR) and 5–90 µgL 1 (SRB). Te calibration of UR and SRB was per- formed separately for each SIS (extracting agent: 0.01 M CaCl2 solution, soil:solution-ratio: 1:5). Samples for the measurement of UR and SRB were diluted 1:10 to reduce DOC background fuorescence and to maintain the − − calibration range. Br and Cl were measured by ion chromatography (790 Personal IC, Deutsche METROHM GmbH & Co KG, Filderstadt, Germany).

Measurement of biocides. Analysis of terbutryn, diuron and OIT was already described in detail in­ 28, 104. For measurement of biocides, 3 mL of the percolates were evaporated to dryness with a Büchi Syncore Polyvap (BÜCHI Labortechnik GmbH, Essen, Germany) and taken up in 0.3 mL acetonitrile (enrichment factor of 10). 90 µL of the percolate sample containing biocides were spiked with 10 µL of terbutryn-D5 as an internal stand- ard. Measurements of percolate samples were conducted with a Triple Quadrupole mass spectrometer (Agilent Technologies, 1200 Infnity LC-System and 6430 Triple Quad, Waldbronn, Germany). Terefore, a NUCLEO- DUR RP-C18 column (125/2 100–3 µL C18 ec; MACHEREY-NAGEL GmbH & Co KG, Düren, Germany) was used as stationary phase, whereas 0.01% formic acid (A) and acetonitrile (B) were used as mobile phases with a − fow of 0.4 mL min 1 and the following concentration gradient: 10% B (0–1 min), 10–50% B (1-11 min), 50–85% B (11–18 min), 85–90% B (18–21 min), 90% B (21–24 min), 90–10% B (24–26 min), 10% B (26–30 min). Oven ◦ temperature was T = 30 C and injection volume 5 µL . Mass spectrometric settings are listed in Table S2. Te − linearity between peak area and concentration of substances were obtained in a range of 0–5 µgL 1 . Hence, lim- its of detection (LOD) and quantitation (LOQ) were calculated with DINTEST (2003) according to DIN 32645 − − − and amounted to 1 and 2.65 µgL 1 (diuron), 0.76 and 2.94 µgL 1 (terbutryn), 0.86 and 2.99 µgL 1 (OIT), respectively. Data availability All data generated or analyzed during this study are included in this published article (and its Supplementary Information fles).

Received: 15 September 2020; Accepted: 15 March 2021

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103. Kasnavia, T., Vu, D. & Sabatini, D. A. Fluorescent dye and media properties afecting sorption and tracer selection. Ground Water 37, 376–381. https://​doi.​org/​10.​1111/j.​1745-​6584.​1999.​tb011​14.x (1999). 104. Hensen, B., Olsson, O. & Kümmerer, K. A strategy for an initial assessment of the ecotoxicological efects of transformation products of pesticides in aquatic systems following a tiered approach. Environ. Int. 137, 105533. https://doi.​ org/​ 10.​ 1016/j.​ envint.​ ​ 2020.​105533 (2020). Acknowledgements We thank Kenton Stutz for his linguistic and technical corrections of the manuscript. Author contributions Conceptualization, methodology: M.B., J.L., M.G.-R., F.L.; investigation, formal analysis, validation and visualiza- tion: M.B., B.H., T.H., E.F.-P.; project administration: J.L., O.O., F.L.; funding acquisition: J.L., F.L., O.O., M.G.-R.; writing—original draf: M.B., B.H.; writing—review and editing: all authors; approval to the fnal version of the manuscript: all authors. Funding Open Access funding enabled and organized by Projekt DEAL. Tis research was funded by the Federal Min- istry of Education and Research (BMBF) (02WRM1366B) in the project MUTReWa (Measures for a sustain- able approach to pesticides and their transformation products in the regional water management) and by the European Union and the European Regional Development Fund (ERDF) within the INTERREG V Upper Rhine program in the project 5.3 NAVEBGO (Sustainable reduction of biocide inputs to groundwater in the Upper Rhine region).

Competing interests Te authors declare no competing interests. Additional information Supplementary Information Te online version contains supplementary material available at https://​doi.​org/​ 10.​1038/​s41598-​021-​86387-9. Correspondence and requests for materials should be addressed to M.B. Reprints and permissions information is available at www.nature.com/reprints. Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional afliations. Open Access Tis article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. Te images or other third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://​creat​iveco​mmons.​org/​licen​ses/​by/4.​0/.

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